26. Dynamic language support

26.1 Introduction

Why only these languages?

The supported languages were chosen because a) the languages
have a lot of traction in the Java enterprise community, b) no requests were made
for other languages within the Spring 2.0 development timeframe, and
c) the Spring developers were most familiar with them.

There is nothing stopping the inclusion of further languages though. If you want
to see support for <insert your favourite dynamic language here>,
you can always raise an issue on Spring's
JIRA
page (or implement such support yourself).

Spring 2.0 introduces comprehensive support for using classes and objects that have
been defined using a dynamic language (such as JRuby) with Spring.
This support allows you to write any number of classes in a supported dynamic language,
and have the Spring container transparently instantiate, configure and dependency inject the
resulting objects.

The dynamic languages currently supported are:

JRuby 0.9 / 1.0

Groovy 1.0 / 1.5

BeanShell 2.0

Fully working examples of where this dynamic language support can be immediately useful
are described in Section 26.4, “Scenarios”.

Note: Only the specific versions as listed above are supported
in Spring 2.5. In particular, JRuby 1.1 (which introduced many incompatible API
changes) is not supported at this point of time.

26.2 A first example

This bulk of this chapter is concerned with describing the dynamic language support
in detail. Before diving into all of the ins and outs of the dynamic language support,
let's look at a quick example of a bean defined in a dynamic language.
The dynamic language for this first bean is Groovy (the basis of this example
was taken from the Spring test suite, so if you want to see equivalent examples
in any of the other supported languages, take a look at the source code).

Find below the Messenger interface that the
Groovy bean is going to be implementing, and note that this interface is defined
in plain Java. Dependent objects that are injected with a reference to the
Messenger won't know that the underlying
implementation is a Groovy script.

Finally, here are the bean definitions that will effect the injection of the
Groovy-defined Messenger implementation into
an instance of the DefaultBookingService class.

Note

To use the custom dynamic language tags to define dynamic-language-backed beans,
you need to have the XML Schema preamble at the top of your Spring XML
configuration file. You also need to be using a Spring
ApplicationContext implementation as your
IoC container. Using the dynamic-language-backed beans with a plain
BeanFactory implementation is supported,
but you have to manage the plumbing of the Spring internals to do so.

The bookingService bean (a
DefaultBookingService) can now use its private
messenger member variable as normal because the
Messenger instance that was injected
into it is a Messenger
instance. There is nothing special going on here, just plain Java and
plain Groovy.

Hopefully the above XML snippet is self-explanatory, but don't worry
unduly if it isn't. Keep reading for the in-depth detail on the whys
and wherefores of the above configuration.

26.3 Defining beans that are backed by dynamic languages

This section describes exactly how you define Spring managed beans in
any of the supported dynamic languages.

Please note that this chapter does not attempt to explain the syntax and
idioms of the supported dynamic languages. For example, if you want to
use Groovy to write certain of the classes in your application, then the
assumption is that you already know Groovy. If you need further details
about the dynamic languages themselves, please consult
Section 26.6, “Further Resources” at the end of this chapter.

26.3.1 Common concepts

The steps involved in using dynamic-language-backed beans are as follows:

Write the test for the dynamic language source code (naturally)

Then write the dynamic language source code itself :)

Define your dynamic-language-backed beans using the appropriate
<lang:language/> element in the XML
configuration (you can of course define such beans programmatically
using the Spring API - although you will have to consult the source
code for directions on how to do this as this type of advanced
configuration is not covered in this chapter). Note this is an iterative
step. You will need at least one bean definition per dynamic
language source file (although the same dynamic language source
file can of course be referenced by multiple bean definitions).

The first two steps (testing and writing your dynamic language source files)
are beyond the scope of this chapter. Refer to the language specification
and / or reference manual for your chosen dynamic language and crack on with
developing your dynamic language source files. You will
first want to read the rest of this chapter though, as Spring's dynamic language
support does make some (small) assumptions about the contents of your dynamic
language source files.

26.3.1.1 The <lang:language/> element

XML Schema

All of the configuration examples in this chapter make use of the
new XML Schema support that was added in Spring 2.0.

It is possible to forego the use of XML Schema and stick with the old-style
DTD based validation of your Spring XML files, but then you lose out
on the convenience offered by the <lang:language/>
element. See the Spring test suite for examples of the older style
configuration that doesn't require XML Schema-based validation
(it is quite verbose and doesn't hide any of the underlying Spring
implementation from you).

The final step involves defining dynamic-language-backed bean definitions,
one for each bean that you want to configure (this is no different from
normal JavaBean configuration). However, instead of specifying the
fully qualified classname of the class that is to be instantiated and
configured by the container, you use the <lang:language/>
element to define the dynamic language-backed bean.

Each of the supported languages has a corresponding
<lang:language/> element:

<lang:jruby/> (JRuby)

<lang:groovy/> (Groovy)

<lang:bsh/> (BeanShell)

The exact attributes and child elements that are available for
configuration depends on exactly which language the bean has been
defined in (the language-specific sections below provide the full
lowdown on this).

26.3.1.2 Refreshable beans

One of the (if not the) most compelling value adds
of the dynamic language support in Spring is the
'refreshable bean' feature.

A refreshable bean is a dynamic-language-backed bean that with a small
amount of configuration, a dynamic-language-backed bean can monitor
changes in its underlying source file resource, and then reload itself
when the dynamic language source file is changed (for example when a
developer edits and saves changes to the file on the filesystem).

This allows a developer to deploy any number of dynamic language source
files as part of an application, configure the Spring container to create
beans backed by dynamic language source files (using the mechanisms
described in this chapter), and then later, as requirements change or
some other external factor comes into play, simply edit a dynamic language
source file and have any change they make reflected in the bean that is
backed by the changed dynamic language source file. There is no need to
shut down a running application (or redeploy in the case of a web application).
The dynamic-language-backed bean so amended will pick up the new state
and logic from the changed dynamic language source file.

Note

Please note that this feature is off by default.

Let's take a look at an example to see just how easy it is to start using
refreshable beans. To turn on the refreshable beans
feature, you simply have to specify exactly one
additional attribute on the <lang:language/> element
of your bean definition. So if we stick with
the example from earlier
in this chapter, here's what we would change in the Spring XML configuration
to effect refreshable beans:

<beans>
<!-- this bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute -->
<lang:groovyid="messenger"refresh-check-delay="5000"<!--switchesrefreshingonwith5secondsbetweenchecks-->script-source="classpath:Messenger.groovy">
<lang:propertyname="message"value="I Can Do The Frug" />
</lang:groovy>
<beanid="bookingService"class="x.y.DefaultBookingService">
<propertyname="messenger"ref="messenger" />
</bean>
</beans>

That really is all you have to do. The 'refresh-check-delay'
attribute defined on the 'messenger' bean definition
is the number of milliseconds after which the bean will be refreshed with
any changes made to the underlying dynamic language source file.
You can turn off the refresh behavior by assigning a negative value
to the 'refresh-check-delay' attribute.
Remember that, by default, the refresh behavior is disabled. If you don't
want the refresh behavior, then simply don't define the attribute.

If we then run the following application we can exercise the refreshable feature;
please do excuse the 'jumping-through-hoops-to-pause-the-execution'
shenanigans in this next slice of code. The System.in.read()
call is only there so that the execution of the program pauses while I (the author)
go off and edit the underlying dynamic language source file so that the refresh will
trigger on the dynamic-language-backed bean when the program resumes execution.

Let's assume then, for the purposes of this example, that all
calls to the getMessage() method of
Messenger implementations have to be
changed such that the message is surrounded by quotes.
Below are the changes that I (the author) make to the
Messenger.groovy source file when the execution of
the program is paused.

When the program executes, the output before the input pause will be
I Can Do The Frug. After the change
to the source file is made and saved, and the program resumes execution,
the result of calling the getMessage() method on the
dynamic-language-backed Messenger implementation
will be 'I Can Do The Frug' (notice
the inclusion of the additional quotes).

It is important to understand that changes to a script will
not trigger a refresh if the changes occur
within the window of the 'refresh-check-delay' value.
It is equally important to understand that changes to the script are
not actually 'picked up' until a method is called
on the dynamic-language-backed bean. It is only when a method is called on a
dynamic-language-backed bean that it checks to see if its underlying script
source has changed. Any exceptions relating to refreshing the script
(such as encountering a compilation error, or finding that the script
file has been deleted) will result in a fatal
exception being propagated to the calling code.

The refreshable bean behavior described above does
not apply to dynamic language source files
defined using the <lang:inline-script/> element
notation (see Section 26.3.1.3, “Inline dynamic language source files”).
Additionally, it only applies to beans where
changes to the underlying source file can actually be detected;
for example, by code that checks the last modified date of a
dynamic language source file that exists on the filesystem.

26.3.1.3 Inline dynamic language source files

The dynamic language support can also cater for dynamic language
source files that are embedded directly in Spring bean definitions.
More specifically, the <lang:inline-script/>
element allows you to define dynamic language source immediately
inside a Spring configuration file. An example will perhaps make the
inline script feature crystal clear:

If we put to one side the issues surrounding whether it is good practice
to define dynamic language source inside a Spring configuration file, the
<lang:inline-script/> element can be useful in
some scenarios. For instance, we might want to quickly add a Spring
Validator implementation to a Spring MVC
Controller. This is but a moment's work
using inline source. (See Section 26.4.2, “Scripted Validators”
for such an example.)

Find below an example of defining the source for a JRuby-based bean
directly in a Spring XML configuration file using the
inline: notation. (Notice the use of the &lt;
characters to denote a '<' character. In such a case
surrounding the inline source in a <![CDATA[]]> region might be better.)

26.3.1.4 Understanding Constructor Injection in the context of dynamic-language-backed beans

There is one very important thing to be aware of
with regard to Spring's dynamic language support. Namely, it is not (currently)
possible to supply constructor arguments to dynamic-language-backed beans (and hence
constructor-injection is not available for dynamic-language-backed beans).
In the interests of making this special handling of constructors and
properties 100% clear, the following mixture of code and configuration
will not work.

<lang:groovyid="badMessenger"script-source="classpath:Messenger.groovy">
<!-- this next constructor argument will *not* be injected into the GroovyMessenger -->
<!-- in fact, this isn't even allowed according to the schema -->
<constructor-argvalue="This will *not* work" />
<!-- only property values are injected into the dynamic-language-backed object -->
<lang:propertyname="anotherMessage"value="Passed straight through to the dynamic-language-backed object" />
</lang>

In practice this limitation is not as significant as it first appears since
setter injection is the injection style favored by the overwhelming majority
of developers anyway (let's leave the discussion as to whether that is a good
thing to another day).

26.3.2 JRuby beans

The JRuby library dependencies

The JRuby scripting support in Spring requires the following
libraries to be on the classpath of your application.
(The versions listed just happen to be the versions that the
Spring team used in the development of the JRuby scripting support;
you may well be able to use another version of a specific library.)

jruby.jar

cglib-nodep-2.1_3.jar

From the JRuby homepage...

“JRuby is an 100% pure-Java implementation of the Ruby programming language.”

In keeping with the Spring philosophy of offering choice, Spring's
dynamic language support also supports beans defined in the JRuby
language. The JRuby language is based on the quite intuitive
Ruby language, and has support for inline regular expressions, blocks
(closures), and a whole host of other features that do make solutions
for some domain problems a whole lot easier to develop.

The implementation of the JRuby dynamic language support in Spring is
interesting in that what happens is this: Spring creates a JDK dynamic
proxy implementing all of the interfaces that are specified in the
'script-interfaces' attribute value of the
<lang:ruby> element (this is why
you must supply at least one interface in the value
of the attribute, and (accordingly) program to interfaces when using
JRuby-backed beans).

Let us look at a fully working example of using a JRuby-based bean. Here is
the JRuby implementation of the Messenger
interface that was defined earlier in this chapter (for your convenience it
is repeated below).

require 'java'
class RubyMessenger
include org.springframework.scripting.Messenger
def setMessage(message)
@@message = message
end
def getMessage
@@message
end
end
# this last line is not essential (but see below)
RubyMessenger.new

And here is the Spring XML that defines an instance of the
RubyMessenger JRuby bean.

Take note of the last line of that JRuby source ('RubyMessenger.new').
When using JRuby in the context of Spring's dynamic language support, you are encouraged
to instantiate and return a new instance of the JRuby class that you want to use as a
dynamic-language-backed bean as the result of the execution of your JRuby source. You
can achieve this by simply instantiating a new instance of your JRuby class on the last
line of the source file like so:

require 'java'
include_class 'org.springframework.scripting.Messenger'
# class definition same as above...
# instantiate and return a new instance of the RubyMessenger class
RubyMessenger.new

If you forget to do this, it is not the end of the world; this will however result in
Spring having to trawl (reflectively) through the type representation of your JRuby class
looking for a class to instantiate. In the grand scheme of things this will be so fast
that you'll never notice it, but it is something that can be avoided by simply
having a line such as the one above as the last line of your JRuby script. If you don't
supply such a line, or if Spring cannot find a JRuby class in your script to instantiate
then an opaque ScriptCompilationException
will be thrown immediately after the source is executed by the JRuby
interpreter. The key text that identifies this as the root cause of an
exception can be found immediately below (so if your Spring container
throws the following exception when creating your dynamic-language-backed bean
and the following text is there in the corresponding stacktrace, this will hopefully
allow you to identify and then easily rectify the issue):

To rectify this, simply instantiate a new instance of whichever class
you want to expose as a JRuby-dynamic-language-backed bean (as shown above). Please
also note that you can actually define as many classes and objects
as you want in your JRuby script; what is important is that the
source file as a whole must return an object (for Spring to configure).

26.3.3 Groovy beans

The Groovy scripting support in Spring requires the following
libraries to be on the classpath of your application.

groovy-1.5.5.jar

asm-2.2.2.jar

antlr-2.7.6.jar

From the Groovy homepage...

“Groovy is an agile dynamic language for the Java 2 Platform that has
many of the features that people like so much in languages like Python, Ruby
and Smalltalk, making them available to Java developers using a Java-like syntax.
”

If you have read this chapter straight from the top, you will already have
seen an example of a
Groovy-dynamic-language-backed bean. Let's look at another example (again
using an example from the Spring test suite).

The resulting output from running the above program will be
(unsurprisingly) 10.
(Exciting example, huh? Remember that the intent is to illustrate the
concept. Please consult the dynamic language showcase project for a
more complex example, or indeed Section 26.4, “Scenarios”
later in this chapter).

It is important that you do not define more than one
class per Groovy source file. While this is perfectly legal in Groovy, it
is (arguably) a bad practice: in the interests of a consistent approach,
you should (in the opinion of this author) respect the standard Java
conventions of one (public) class per source file.

26.3.3.1 Customising Groovy objects via a callback

The GroovyObjectCustomizer
interface is a callback that allows you to hook additional
creation logic into the process of creating a Groovy-backed bean.
For example, implementations of this interface could invoke
any required initialization method(s), or set some default property
values, or specify a custom MetaClass.

The Spring Framework will instantiate an instance of your Groovy-backed
bean, and will then pass the created GroovyObject
to the specified GroovyObjectCustomizer
if one has been defined. You can do whatever you like with the supplied
GroovyObject reference: it is expected
that the setting of a custom MetaClass is what most
folks will want to do with this callback, and you can see an example
of doing that below.

A full discussion of meta-programming in Groovy is beyond the scope of the
Spring reference manual. Consult the relevant section of the Groovy
reference manual, or do a search online: there are plenty of articles
concerning this topic.
Actually making use of a GroovyObjectCustomizer
is easy if you are using the Spring 2.0 namespace support.

<!-- define the GroovyObjectCustomizer just like any other bean -->
<beanid="tracingCustomizer"class="example.SimpleMethodTracingCustomizer" />
<!-- ... and plug it into the desired Groovy bean via the 'customizer-ref' attribute -->
<lang:groovyid="calculator"script-source="classpath:org/springframework/scripting/groovy/Calculator.groovy"customizer-ref="tracingCustomizer" />

If you are not using the Spring 2.0 namespace support, you can still
use the GroovyObjectCustomizer functionality.

26.3.4 BeanShell beans

The BeanShell scripting support in Spring requires the following
libraries to be on the classpath of your application.

bsh-2.0b4.jar

cglib-nodep-2.1_3.jar

All of these libraries are available in the Spring-with-dependencies
distribution of Spring (in addition to also being freely available
on the web).

From the BeanShell homepage...

“BeanShell is a small, free, embeddable Java source interpreter
with dynamic language features, written in Java. BeanShell dynamically
executes standard Java syntax and extends it with common scripting
conveniences such as loose types, commands, and method closures like those
in Perl and JavaScript.”

In contrast to Groovy, BeanShell-backed bean definitions require some (small)
additional configuration. The implementation of the BeanShell dynamic language
support in Spring is interesting in that what happens is this: Spring creates
a JDK dynamic proxy implementing all of the interfaces that are specified in the
'script-interfaces' attribute value of the
<lang:bsh> element (this is why
you must supply at least one interface in the value
of the attribute, and (accordingly) program to interfaces when using
BeanShell-backed beans). This means that every method call on a BeanShell-backed
object is going through the JDK dynamic proxy invocation mechanism.

Let's look at a fully working example of using a BeanShell-based bean
that implements the Messenger interface
that was defined earlier in this chapter (repeated below for your
convenience).

26.4 Scenarios

The possible scenarios where defining Spring managed beans in a scripting
language would be beneficial are, of course, many and varied. This section
describes two possible use cases for the dynamic language support in Spring.

26.4.1 Scripted Spring MVC Controllers

One group of classes that may benefit from using dynamic-language-backed
beans is that of Spring MVC controllers. In pure Spring MVC applications,
the navigational flow through a web application is to a large extent
determined by code encapsulated within your Spring MVC controllers.
As the navigational flow and other presentation layer logic of a web
application needs to be updated to respond to support issues or changing
business requirements, it may well be easier to effect any such required
changes by editing one or more dynamic language source files and seeing
those changes being immediately reflected in the state of a running
application.

Remember that in the lightweight architectural model espoused by projects
such as Spring, you are typically aiming to have a really
thin presentation layer, with all the meaty business
logic of an application being contained in the domain and service layer
classes. Developing Spring MVC controllers as dynamic-language-backed beans
allows you to change presentation layer logic by simply editing and saving
text files; any changes to such dynamic language source files will (depending
on the configuration) automatically be reflected in the beans that are backed
by dynamic language source files.

Note

In order to effect this automatic 'pickup' of any changes
to dynamic-language-backed beans, you will have had to enable the
'refreshable beans' functionality. See
Section 26.3.1.2, “Refreshable beans” for a full treatment
of this feature.

Find below an example of an
org.springframework.web.servlet.mvc.Controller
implemented using the Groovy dynamic language.

26.4.2 Scripted Validators

Another area of application development with Spring that may benefit
from the flexibility afforded by dynamic-language-backed beans is that of
validation. It may be easier to express complex validation
logic using a loosely typed dynamic language (that may also have support
for inline regular expressions) as opposed to regular Java.

Again, developing validators as dynamic-language-backed beans allows you to change
validation logic by simply editing and saving a simple text file; any such
changes will (depending on the configuration) automatically be reflected
in the execution of a running application and would not require the restart
of an application.

Note

Please note that in order to effect the automatic 'pickup' of any changes
to dynamic-language-backed beans, you will have had to enable the
'refreshable beans' feature. See
Section 26.3.1.2, “Refreshable beans” for a full and
detailed treatment of this feature.

26.5 Bits and bobs

This last section contains some bits and bobs related to the dynamic language
support.

26.5.1 AOP - advising scripted beans

It is possible to use the Spring AOP framework to advise scripted beans.
The Spring AOP framework actually is unaware that a bean that is being
advised might be a scripted bean, so all of the AOP use cases and functionality
that you may be using or aim to use will work with scripted beans. There is
just one (small) thing that you need to be aware of when advising scripted
beans... you cannot use class-based proxies, you must use
interface-based proxies.

You are of course not just limited to advising scripted beans... you can
also write aspects themselves in a supported dynamic language and use such
beans to advise other Spring beans. This really would be an advanced use of
the dynamic language support though.

26.5.2 Scoping

In case it is not immediately obvious, scripted beans can of course be scoped
just like any other bean. The scope attribute on the
various <lang:language/> elements allows you to
control the scope of the underlying scripted bean, just as it does with a
regular bean. (The default scope is
singleton, just as it
is with 'regular' beans.)

Find below an example of using the scope attribute
to define a Groovy bean scoped as a
prototype.

26.6 Further Resources

Some of the more active members of the Spring community have also added support for
a number of additional dynamic languages above and beyond the ones covered in this
chapter. While it is possible that such third party contributions may be added to the
list of languages supported by the main Spring distribution, your best bet for seeing
if your favourite scripting language is supported is the
Spring Modules project.